COMPOSITION OF THE INSECT-CRUST. 13 



needed by the insect to perform its complicated motions 

 while on the wing. 



The insertion of the fore wing is concealed by the "shoulder 

 tippets," or patagia (Fig. 11), which are only present in the 

 mesothorax. The external opening of the spiracles just under 

 the wing perforates a little piece called by Audouin the peri- 

 treme, 



A glance at Figures 11 and 12 shows how compactly the 

 various parts of the thorax are agglutinated into a globular 

 mass, and that this is due to the diminished size of the first 

 and third rings, while the middle ring is greatly enlarged to 

 support the muscles of flight. There are four tergal, four 

 pleural, two on each side (and these in the Hymenoptera, Lepi- 

 doptera, and Diptera subdivide into several pieces), and a 

 single sternal piece, making nine for each ring and twenty- 

 seven for the whole thorax, with eight accessory pieces (the 

 three pairs of peritremes and the two patagia), making a total 

 of thirty-five for the entire thorax ; or, multiplying the four 

 tergal pieces by two, since they are formed by the union of two 

 primitive pieces on the median line of the body, we have 

 thirty-nine pieces composing the thorax. 



TABLE OF THE PARTS OF THE THORAX APPLIED TO THE PRO-, 

 MESO-, AND METATIIORAX, RESPECTIVELY. 



, Pnescutum, 

 fDorsnl S Scutum, 

 I Surface J Scutcllum. 



- Postscutellum. 



Thorax P.c,,,,, 



i w 1 1 vf n r* o 



' ( Episterual apophysis, Stigma, Peritreme. 



We must remember that these pieces are rarely of precisely 

 the same form in any two species, and that they differ, often in 

 a very marked way, in different genera of insects. How sim- 

 ple, then, is the typical ring, and how complex are the va- 

 rious subdivisions of that ring as seen in the actual, living 

 insect, where each part has its appropriate muscles, nerves, and 

 tracheae ! 



We have seen how the thorax is formed in Insects generally, 

 let us now advert to the two types of thorax in the six-footed 



